The present invention relates to a process for making aromatic bis(etherimide)s, based on the reaction between an aromatic bisphenoxide salt and a nitro-substituted phthalimide in the presence of a non-polar organic solvent and a phase transfer catalyst. More particularly, the present invention relates to the use of a mixture of a dipolar aprotic solvent and a zinc salt as a phase transfer catalyst for making aromatic bis(etherimide) using a non-polar organic solvent.
Phase transfer catalysis is a method commonly practiced for conducting reactions in non-polar aprotic solvents when one or more of the reagents required for reaction is/are in the insoluble solid phase and the same is/are in contact with a solution of other reagent(s) of interest.
Typically crown ethers, cryptands and quaternary ammonium or phosphonium salts are used as phase transfer catalysts for the purpose of transferring the insoluble solid reagent(s) into the liquid phase in which the reaction takes place.
Although crown ethers and cryptands are thermally stable, they are quite expensive and in many cases are poor phase transfer catalysts for the desired reaction.
Quaternary ammonium and phosphonium salts, although less expensive than crown ethers and cryptands, are still relatively expensive catalysts which require recycling for cost effective utilization. Further, the quaternary salts tend to be thermally unstable, which leads to catalyst losses via decomposition and contamination of the desired product with by-products from the decomposition process.
The use of dipolar aprotic solvents as the reaction medium for effecting reaction between aromatic bisphenoxide alkali metal salt and N-alkyl nitrophthalimide is also well known, as is the use of cosolvent mixtures, e.g., as much as 80% toluene with 20% dipolar aprotic solvent. The use of such a cosolvent mixture can be moderately effective; however, when 80% or more toluene is used, the yield of product is quite low. For example, using a 90:10 (w/w) ratio of toluene to dimethyl sulfoxide, less than 30% of the desired aromatic bis(ether phthalimide) is obtained after 90 minutes at reflux.
In particular instances, a non-polar solvent, such as toluene can be used with a phase transfer catalyst, such as a tetra-ammonia salt to effect displacement of reactive radicals on a phthalimide nucleus with a mono, or bisalkali metal phenoxide to make aromatic etherimides, as shown by Williams, U.S. Pat. No. 4,273,712, assigned to the same assignee as the present invention. Additional methods for making aromatic bis(etherimide)s using phase transfer catalysts, as shown by Williams et al, U.S. Pat. No. 4,257,953 and Relles et al U.S. Pat. No. 4,247,464. Although valuable results are achieved by using such tetra-ammonia salts as phase transfer catalysts for aromatic bis(etherimide) production, alternative procedures are constantly being sought by those skilled in the art.
The present invention is based on our discovery that aromatic bis(ether phthalimide)s of the formula ##STR1## can be made by effecting reaction between an alkali metal bisphenoxide of the formula EQU X.sup.1 --O--R.sup.1 --O--X.sup.2 ( 2)
and a nitro-substituted N-alkyl phthalimide or N-alkyl phthalimide of the formula ##STR2## in the presence of a non-polar organic solvent and an effective amount of a phase transfer catalyst in the form of a mixture of a dipolar aprotic solvent and a zinc halide, for example, zinc chloride, where R is a radical selected from hydrogen, a C.sub.(1-8) alkyl radical, a C.sub.(6-13) aryl radical and mixtures thereof, and R.sup.1 is a C.sub.(6-30) aromatic organic radical, and X.sup.1 and X.sup.2 can be the same or different alkali metal ions selected from sodium, potassium, cesium and preferably sodium.